The present invention relates to switched-mode power supplies. More particularly, the present invention relates to a device and method for providing a regulated self-supply power source for switched-mode power supplies.
Most modern electronic devices employ switched-mode power supplies due to the high power efficiency and smaller physical size of these types of power supplies. Switched-mode power supplies are also advantageous in that these supplies can provide regulated output voltages having magnitudes higher or lower than the unregulated input supply, and also provide multiple outputs of differing voltage magnitudes.
Switched-mode supply designs are varied, and one of the most common designs is the so-called flyback power supply. One typical flyback power supply includes, in addition to the general circuitry, an auxiliary circuit which is used to supply operational power to the power supply controller circuitry.
There are various weaknesses associated with the use of an auxiliary circuit to supply the controller. First, the auxiliary circuit adds cost and complexity. Second, it adds size to the overall power supply circuit due to the added bulk of the auxiliary winding and its associated components. Third, the auxiliary winding voltage will vary with the supply operating conditions. And last, the auxiliary circuit doesn""t provide sufficient operating power if the power supply output is ever overloaded.
Of course, other power supply designs, in addition to the flyback design just described, also employ auxiliary circuits. These other power supply designs suffer the same weaknesses associated with the use of the auxiliary circuit, as the flyback design.
Another flyback power supply design alleviates the need for the auxiliary circuit. This design includes an integrated self-supply circuit for providing operational power to, among other circuitry, the controller circuit. With this design, a power source, such as a constant current source, is selectively connected to charge a capacitor at a predetermined frequency, to a predetermined voltage threshold, and a duty cycle that varies with loading conditions. This operation is controlled by an oscillator operating at a frequency that is typically between 20 kHz and 150 kHz. The oscillator frequency is set such that the capacitor discharges only a few millivolts, typically around 10 to 50 millivolts. This fairly tight line regulation provides for a robust analog circuit design.
Although the above-described design provides a robust design, there are also weaknesses associated with this design. One weakness is that the oscillator frequency is set high enough for undesirable electromagnetic interference (EMI) emissions to occur from the power supply. Also, significant power dissipation occurs with this design because of switching losses, which are proportional to operating frequency. This method also requires additional circuitry to prevent an overvoltage condition due to its fixed, high frequency operation and minimum duty cycle due to delays.
Thus, there is a need in the art for a device that dynamically utilizes power from the primary side of a switched-mode power supply to supply power to the power supply""s primary side controller without requiring the need for an auxiliary winding and associated circuitry and the inherent drawbacks discussed above. There is also a need for such a device that does not require such tight line regulation and therefore does not require a high frequency oscillator and high frequency operation of the power management switch, as well as the cost of additional circuitry.